Rates of 1,5-Hydrogen Abstraction in 1,1,2,2-Tetrafluoro-<i>n</i>-pentyl and 1,1,2,2,3,3-Hexafluoro-<i>n</i>-pentyl Radicals

A new competitive kinetic method to obtain the relative rates of intramolecular hydrogen abstraction versus intermolecular deuterium abstraction has been developed and used to measure the rates of the relatively slow unimolecular 1,5-hydrogen abstractions by 1,1,2,2-tetrafluoroalkyl (XCH2CH2CH2CF2CF2•) and 1,1,2,2,3,3-hexafluoroalkyl (XCH2CH2CF2CF2CF2•) radicals. Substituents at the site of C−H abstraction (X = CH3O−, CH3S−, CH3−, C6H5−, CH3C(O)O−) have only a moderate effect, with the rate constants for 1,5-shifts in the 103 s-1 range. Values of kH and kD for bimolecular hydrogen/deuterium transfer from t-BuMe2SiH(D) and Me3SiSiMe2H(D) are reported along with measures of side-chain H transfer from the deuterated silanes

Bimolecular Kinetic Studies with High-Temperature Gas-Phase ¹⁹F NMR: Cycloaddition Reactions of Fluoroolefins^†

A gas-phase NMR kinetic technique has been used for the first time to obtain accurate measurements of rate constants of some bimolecular, second-order cycloaddition reactions. As a test of the potential use of this technique for the study of second-order reactions, the rate constants and the activation parameters for the cyclodimerization reactions of chlorotrifluoroethylene (CTFE) and tetrafluoroethylene (TFE) were determined in the temperature range 240−340 °C, using a commercial high-temperature NMR probe. Obtaining excellent agreement of the results with published data, the technique was then applied to the reaction of 1,1-difluoroallene with 1,3-butadiene, the results of which indicate that the use of gas-phase NMR for reaction kinetics is particularly valuable when a reagent is available only in small amounts and in cases where there are several competing processes occurring simultaneously. The major processes observed in this reaction are regioselective [2+2] and [2+4] cycloadditions, whose rates and activation parameters were determined [k2 = 9.3 × 106 exp(−20.1 kcal mol-1/RT) L/mol-1 s-1 and k3 = 1.2 × 106 exp(−18.4 kcal mol-1/RT) L/mol-1 s-1, respectively] in the temperature range 130−210 °C

Absolute Rates of Intermolecular Carbon−Hydrogen Abstraction Reactions by Fluorinated Radicals^§

Using competition kinetic methodology, absolute rate constants for bimolecular hydrogen abstraction from a variety of organic substrates in solution have been obtained for the n-C4H9CF2CF2•, n-C4F9•, and i-C3F7• radicals. Fluorine substitution substantially increases the reactivity of alkyl radicals with respect to C−H abstraction, with the secondary radical being most reactive. A wide range of substrate reactivities (5200-fold) was observed, with the results being discussed in terms of an interplay of thermodynamic, polar, steric, stereoelectronic, and electrostatic/field effects on the various C−H abstraction transition states. Representative carbon−hydrogen bond dissociation energies of a number of ethers and alcohols have been calculated using DFT methodology

Rate of Cyclization of Perfluoro-4-Oxa-5-hexenyl Radical. Use of Tributylgermanium Hydride as an Effective H-Transfer Agent for Perfluoro-<i>n</i>-alkenyl Radicals^†

Rate of Cyclization of Perfluoro-4-Oxa-5-hexenyl Radical. Use of Tributylgermanium Hydride as an Effective H-Transfer Agent for Perfluoro-n-alkenyl Radicals†</sup

Remarkable Cyclization Reactivities of Partially-Fluorinated 6-Heptenyl Radicals^†

Remarkable Cyclization Reactivities of Partially-Fluorinated 6-Heptenyl Radicals†</sup

Cyclizations of 5-Hexenyl, 6-Heptenyl, 7-Octenyl, and 8-Nonenyl Radicals. The Kinetic and Regiochemical Impact of Fluorine and Oxygen Substituents^‖

Using competition kinetic methodology, rate constants for cyclizations of a series of hydrofluorocarbon (HFC) and ether 5-hexenyl, 6-heptenyl, and 7-octenyl radicals have been determined. Remarkably large rate constants (>107 s-1) have been observed for 6-exo-cyclizations of 1,1,2,2-tetrafluoro- and 1,1,2,2,3,3,4,4,-octafluoro-6-heptenyl radicals (>103 those of analogous hydrocarbon radicals), whereas HFC hexenyl and heptenyl ethers exhibit lower cyclization reactivity, as do HFC 7-octenyl radical systems, which cyclize in an endo manner. HFC 8-nonenyl radicals were not observed to cyclize. The results can be rationalized in terms of transition state polar influences, though other factors may also play significant roles

Cyclizations of 5-Hexenyl, 6-Heptenyl, 7-Octenyl, and 8-Nonenyl Radicals. The Kinetic and Regiochemical Impact of Fluorine and Oxygen Substituents^‖

Using competition kinetic methodology, rate constants for cyclizations of a series of hydrofluorocarbon (HFC) and ether 5-hexenyl, 6-heptenyl, and 7-octenyl radicals have been determined. Remarkably large rate constants (>107 s-1) have been observed for 6-exo-cyclizations of 1,1,2,2-tetrafluoro- and 1,1,2,2,3,3,4,4,-octafluoro-6-heptenyl radicals (>103 those of analogous hydrocarbon radicals), whereas HFC hexenyl and heptenyl ethers exhibit lower cyclization reactivity, as do HFC 7-octenyl radical systems, which cyclize in an endo manner. HFC 8-nonenyl radicals were not observed to cyclize. The results can be rationalized in terms of transition state polar influences, though other factors may also play significant roles